Electrostatic loud-speaker



April l, 1930. G, sElBT ELEGTROSTATIG LOUDx SPEAKER Filed Aug. 8, 1927 Patented pr. l, 1930 IED STATES GEORG SEIBT, O BERLIN-SCHONEBERG, GERMANY ELECTROSTATIC LOUD-SPEAKER Application led August 8, 1927, Serial No. 211,544, and` in Germany August 12, 1926.Y

This invention relates to a loud speaker operating on the electrostatic principle. Its main characteristic consists in the fact that those surfaces or surface elements which are 5 set in oscillation by the 'electromechanical forces, and which emit the sound into the surrounding space, are arranged in a manner which is particularly advantageous for the radiation of sound.

VAccording to the invention the arrangement is such that the propa ation of sound can take place in as many di erent directions as may be desired. This is attained by constructing the xed electrode in the form of a 15 prism, the outer faces of which are wholly or partly covered by the vibrating coverings. In order not to damp the vibrations of the diaphragm more than is necessar the prism is preferably made hollow an the faces 20 covered by the diaphragm are perforated in a manner known in itself, these perforations reventin an excessive air-cushioning eifect etween t 'e diaphragm and the fixed electrede. i

The number of sides of the prism may be as large as may be desired. Prisms of hexagonal or quadrilateral shape are advantageous. III

the latter case the prism may assume the special form of a cube.

IThe invention is illustrated by way of example the accompanying drawings, wherem:-

Figures 1 and 2 show in cross section and in plan respectivel one particularly advant eous form of e invention;

imite 3 is a plan and Figure 4 a corre- Y sponding sectional elevation of another constructional form of the invention; and l '50 of convenience in manufacture it is owever Figure 5 shows an outer covering of thev o loud advisable to mill, plane or turn channels or grooves in the brass body. If the grooves are planed they will run in the longitudinal direction of the prism. If they are turned they will run circumferentially round the periphery.

Now round the periphery of the prism is placed a suitable diaphragm M of indiarubber, Celluloid or oiled paper. That side of the diaphragm which is rendered conductive (1), by coating it with colloidal graphite (aquadag) for example, is on the outside, if the prism itself is not insulated on the external periphery.

If such insulating is carriedV out however, for example by coating it with a layer-0f varnish, a metallic diaphragm, of aluminium foil for example can be used, or the layer of aquadag on the Celluloid diaphragm may be turned inwards. In this way the great ad' vantage of smaller air gaps is obtained. The diaphragm is only to bear on the edges of the prism and elsewhere is to be separated by a definite air gap from the metallic body, which may be about 0.03 millimtres at the centre of the sides. In order to secure this air gap, the sides of the polygon are not exactly rectilinear, but are slightly concave in the centre, as shown on an exaggerated scale at a. The departure from the straight shouldamount to about 0.05 millimetres at the centre, and decrease uniformi, towards the edges of the polygon. Now i the diaphragm operates under a direct current voltage, it deflects somewhat, by about 0.02 millimetres at the centre of the sides of the pol gon, so that there still remains there a wor 'ng distance of 0.03 millimetres.

It is not advisable to select for the diaphra a single large coherent surface, since in such a case a uniform bearing on the edges of the prism is not ensured, an the air gaps at those positions where the diaphragm is intended to vibrate are by no means uniform; for it must be observed that dilerences in distance of a few hundredths of a millimetro already produce a considerable eect, as the distance of the diaphragm from the cylinder at those points where it has to vibrate is regularly on an average only a few hundredths of a millimtre, for example 0.03 millimetre.

Irregularities in the air gap make themselves felt by a diminution in the sensitiveness.

'On these grounds the diaphragmv is divided into bands or strips which are passed round the periphery of the cylinder and are connected by their ends with one another or with the cylinder. Fig. 2 shows three bands of Celluloid for example, the ends of which are connected with two small metallic pieces m m, which in their turn are screwed together. By turning the screw to a greater or less extent the pull of the band can be adjusted, which is of importance for the..purpose of bringing out: the utmost sensitivity of the loud speaker.

Instead of bands or strips, thin threads which have been rendered conductive, silk threads for example, may be employed, which have been drawn through aquadag. These threadsare wound onto the cylinder thread by thread, so that as regards. sound emissionthey act exactly like a coherent layer. Of course in such a case the surface of the cylinder or prism must be provided with a coating of varnish in order to prevent short circuits. The sort of varnish which is employed for the so-called varnish wires known in electrical technology is quite suitable.

The silk threads may alternatively be replaced by thin copper or aluminium wires insulated with varnish or silk braiding.

One great practical advantage of the loud speaker described consists in the comparatively shall amount of room it takesup as compared with the statophones with a single large diaphragm.

Obviously the two end faces of the prism may also be covered by the vibrating coverings. 'Fo'r the sake of greater clearness this case is not illustrated in Figures 1 and 2. The covering of all the surfaces with diaphragms comes into consideration more particularly when the prism is of quadrilateral cross section and its height is equal to the length of its sides, fhat i s, when the fixed electrode is of cubical form.

The prism forming the fixed elect-rode may alternatively consist of a very large number of side faces, and in the limit may even be a cylinder. In the latter case it is not the cylinder surface but the base and top surfaces that are covered with the vibrating diaphragms. Such a constructional form is shown in Figures 3 and 4.

Figs. 3 and 4 illustrate one constructlonal example, Fig. 3 being a plan, and Fig. 4 a corresponding sectional elevation. I n these two figures S S are two circular pieces of metal, which are provided with numerous holes of from 1 to 2 millimetres in diameter. The two pieces may be soldered together or otherwise connected together at c. r 1 are rings of insulating material about 5 millimetres wide and 0.05 millimetre thick. M1 M2 are two diaphragms which are lightly stressed at the periphery in a convenient manner. As a material for thel diaphragms indiarubber is suitable, which, on the side remote from the metallic pieces S S is rendered conducting by coating it with graph ite. The intermediate rings 1' 1' may be stuck onto the metallic pieces, and the diaphragms onto therings with adhesive. Since the diaphragms deflect most at the centre, it is advantageous not to turn the metallic pieces S S, exactly plane, but to curve the surfaces slightly, in order that the diaphragms may not bear against them in the centre. Instead of indiarubber, celluloid or some other material which is rendered conducting on one side may be employed for the diaphragm. If metal such as aluminium is used for the diaphragm, it is advisable to varnish the pieces S S on the surface or to coat them with paper, care being taken that the perforations arenot left closed up. Paper readily admits of being subsequently pierced ywith a needle.

The metal pieces S S form one coating, and the two diaphragms the other coating of the condenser loud speaker. The arrangement of the electrical connections and the method of operation do not in any way differ from those which are usual 'with A other electrostatic loud speakers.

Fig. 5 illustrates an outer covering of the Y cylinder `loud speaker according to Figs. 1 and 2, and is intended to show the possibility of an artistic design.

Thus for example the fixed counter-electrode might be constructed as an elongated prism of triangular cross section, as indicated in Figs. 9 and 10.

What I claim is 1. Anelectrostatic loud speaker, comprising a fixed prism, and vibrating coverings covering the external faces of said fixed prism.

2. An electrostatic loud speaker, comprising a fixed hollow prism, said hollow prism being formed with perforations in its faces, and vibrating coverings covering externally the perforated faces o said fixed prism.

3. An electrostatic loud speaker, comprising a fixed prism with its faces made somewhat concave, and vibrating coverings covering the external faces of said fixed prism, the vibrating coverings being electrically insulated from the xed prism where they bear upon said fixed prism.

4. An electrostatic loud speaker, comprising a fixed prism, and vibrating coverings covering the external faces of said fixed prism, said vibrating coverings being made of insulating material and rendered conductive on their external surfaces.

5. An electrostatic loud speaker, comprismeals? a. fixed pri'sm and vibrating coverings covering the external faces of said fixed prism, said vibrating coverings consisting of passed round the peripheral su ace of 't e prism. Y

6r A'method of manufacturing an electro- 'vstatic'lond speaker, comprising milling Hats ,on-'acylinder to produce a fixed rism, and

t covering the external Yfaces of the xed prism so produced with` vibrating coverings.`

7. A n electrostatic loud speaker, comprising a fixed prism, and vibrating coverings covering the external faces of said fixed prism, said vibrating coverings being made of insulating material and rendered conductive on their external surfaces, and said insulating coverings being in contact with said fixed prism.

In. testimony whereof I have signed my name to this specification.

GEORG SEIB'r. 

